Isolation valves are used in subterranean locations for separating one location from another by preventing flow. Some of these devices are safety valves that have the ability to control pressure differential in a direction from below to above. These safety valves have a closure device known as a flapper that is operated by a flow tube that is in turn actuated by a hydraulic piston operated through a hydraulic system controlled at a surface location. In flapper type valves the need to equalize pressure across the flapper when in the closed position has been met with a valve located in the flapper that is first encountered by the flow tube to open a passage through the flapper for pressure equalization before the flow tube pushes the flapper itself to turn 90 degrees to the open position as the flow tube advances past the displaced flapper. Examples of such designs can be seen in U.S. Pat. Nos. 4,478,286; 6,644,408; 6,848,509 and 6,877,564.
Other designs have focused on pressure equalizing across the hydraulic piston that actuates the flow tube in the event there is a seal leak or tubing failure in the control system. In those instances in systems with two control lines there is an equalizing valve in the hydraulic system that can open to put the operating piston in pressure balance so that a closure spring acting on the hydraulic piston pushes up the hydraulic piston and with it the connected flow tube so that the safety valve can close. One example of such a system is U.S. Pat. No. 6,109,351.
The present invention also deals with the concept of pressure equalization across a closed closure member. The reason to equalize pressure across the closure element is to make it possible for the operating system for the closure member to do its job. The control system components do not have to be designed to resist the higher differential pressures which for example can significantly increase seal friction when trying to for example rotate the ball or plug to the open position. There are basically three ways to equalize across a closed valve member before trying to open it. The flow can be equalized either through the member, between the member and one of its seats or between locations on opposed sides of the closed member but spaced apart from the member. In the present invention, the latter option is employed and the normal hydraulic system for opening and closing the valve member is employed in a manner that allows for equalization through passages that are discrete from the hydraulic lines that normally operate the valve member. In essence, in the preferred embodiment, the equalization takes place via the same mechanism that will ultimately open the valve. These and other aspects of the present invention will become more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.